In conventional paint spray guns, a stream of paint under pressure is discharged from a relatively small orifice in a nozzle while air under pressure is discharged radially inwardly into the stream from an annular opening surrounding the nozzle closely adjacent the paint discharge orifice to atomize the stream of paint into a spray of fine particles. The spray produced moves away from the gun in an expanding conical pattern whose apex is at the nozzle.
It is frequently desired to modify the circular cross-section of the normal conical spray pattern by transforming this pattern into one of a narrowed and elongated generally elliptically shaped cross-section so that the spray pattern more closely resembles that of a flat sided fan.
Conventionally, fanning of the spray pattern is accomplished by providing a pair of diametrically opposed ports on the front of the air cap spaced radially outwardly from opposite sides of the annular air discharge opening. The diametrically opposed ports are oriented to direct air jets toward opposite sides of the spray pattern at a location spaced a short distance forwardly from the nozzle orifice. These jets have the effect of flattening the sides of the conical spray pattern against which they are directed. At any given distance from the nozzle, this action transforms the normally circular cross-section of the conical spray into a generally elliptically shaped cross-section. The major axis of the conical spray is somewhat greater than the original cone diameter, and the minor axis of the conical spray is somewhat less than the original cone diameter. The elliptical cross-section becomes more flat with an increase of the air pressure from the diametrically opposed fanning ports.
Conventionally, adjustment of the fanning of the paint spray is accomplished by either rotatably adjusting the air cap (turbine powered low pressure guns) or through an adjustment valve (high pressure guns). A valve stem is adjusted by a thumb screw to restrict the flow of compressed air into a second passage connected to the fanning ports on the air cap. Conventional automatic guns have a separate air supply that can adjust the flow independent of the thumb screw. This adjustment exerts a valving action which establishes maximum air flow when the diametrically opposed valve ports lie in either a vertical plane containing the nozzle axis or a horizontal plane containing the nozzle axis. The flow through the fanning air ports is reduced as the air cap is rotated, and the fanning air flow is cut off when the fanning air ports are midway between the horizontal and vertical positions referred to above. When the fanning air ports are at this midway position, the paint spray assumes its original conical form.
While the foregoing arrangement provides for adjustment of the fanning air to the paint spray, this adjustment is dependent upon the rotated position of the air cap about the nozzle axis. Adjustment of the fan width (minor axis of the elliptical fan cross-section) to a width between maximum or unmodified conical spray and minimum width requires the ports of the air cap to be disposed in a general plane inclined from the vertical. This inclination of the fanning air ports establishes the angle that the major axis of the elliptical configuration will assume with respect to the vertical, a situation which is inconvenient to the operator who would prefer that this major axis be either vertical or horizontal for all degrees of fanning.
The present invention is directed to a spray gun in which fanning may be adjustably controlled independently of the rotative orientation of the air cap and which operates on air being supplied to the paint spray gun at low pressures and high volumes.